A fake or impostor; something or someone that is not genuine or who pretends to be someone/something else.
その時計は高級ブランドの偽者だから買わない方がいいよ。
That watch is a fake of a luxury brand, so you shouldn't buy it.
舞台で彼は王のふりをする偽者の役を演じた。
On stage he played the role of an impostor pretending to be the king.
その話を信じるなんて、君は簡単にだまされる偽者を見抜けない人だね。
To believe that story shows you easily fall for fakes and can't see through impostors.
店で売られていた絵画は偽者と判定された。
The painting sold in the shop was determined to be a fake.